The present invention is directed to improving the stability of oscillating devices in applications such as communication systems which up or down convert signals in the transmitters and/or receivers and in frequency synthesizers where pure and stable frequency references are required. Other applications for the device and method of the present invention are in systems that require highly stable clock signals for reference purposes such as watches and computing devices and the device and method are also useful in any area where oscillators are applied.
Any system in which timing is important in some way has a local oscillator and many systems also require accurate frequency references that are generated locally. For instance, radio would not be possible without a local oscillator in the transmitter and receiver for converting the information signals to the proper frequency bands. The performance of these systems is highly dependent on the stability of the frequency provided by the oscillator. Phase jitter (phase noise) in the output of the oscillator limits the accuracy of the reference and therefore limits the accuracy of the entire system. Amplitude variations in the output of the oscillator are important as well in many applications but these amplitude variations can be readily suppressed with a limiter or an automatic gain control.
The general structure for high performance oscillators includes an amplifier which has its output fed back to its input via a resonating structure. The resonating structure produces a large output change when the frequency in the feedback loop varies and this output change opposes the frequency variations so that the frequency variations are minimized. As the change in the resonator output on a frequency variation becomes larger, the correction also becomes stronger. The resonator change with frequency is indicated by a quality or Q-value. A higher Q-value indicates that the resonator is more sensitive to frequency variations and therefore the ultimate oscillator output frequency will be more stable. In order to provide oscillators having pure and stable frequency outputs, low noise and high Q-values are required for the oscillator. High Q-values in the resonating structures for stable oscillators are essential because the final output spectrum of the oscillator is determined by the noise generated in the loop and the Q-value of the resonator.
High Q-value resonators are typically made with piezoelectric crystals which provide a steep phase change at a specific frequency. Ceramic resonators can also be applied that resonate at a frequency related to their physical dimensions or at a lower frequency where the resonators exhibit inductance characteristics. Furthermore, LC resonators (coils in combination with capacitors) can be used. In addition to the fact that crystal resonators can only be used up to relatively low frequencies, all of these resonator types are not suitable to be integrated with the electronics on an IC chip and these resonator types use discrete components as a result. Accordingly, integration of resonating components is very desirable to provide on-chip references to improve cost, performance and reliability of the devices. However, the only resonator type that may be possibly used for chip integration is the LC structure. Nevertheless, standard IC processes provide low Q-value coils (high series resistance), which limits the Q-value of the LC structure resonator, and therefore, does not allow for integration of high Q-value oscillators.
An oscillator can be modeled as a noise filter with an extremely small bandwidth and therefore the output spectrum of the oscillator can be considered as narrowband noise. This noise can be split into amplitude or AM noise, and phase (PM) or frequency (FM) noise. The AM noise can be suppressed by using an automatic gain control (AGC) or a limiter in the feedback loop. However, FM noise depends on the correction capabilities of the resonator in the feedback loop or on the Q-value of the resonator. When AM noise is removed at the oscillator output, only FM noise remains and the oscillator can be modeled as an ideal oscillator (having an infinite Q-value) with a noisy signal on its frequency control input. Noise at the control input provides an FM modulation of the oscillator output similar to a non-ideal oscillator output.
FIG. 1 illustrates the configuration for a typical oscillator where an amplifier 10 is fed back by a passive, phase rotating network 20. The oscillator 5 starts to oscillates at the frequency for which the Barkhausen conditions are fulfilled where the loop gain is exactly 1 and the loop phase is 0 or 2.pi.. The oscillator 5 is preferably designed such that the phase condition occurs in the steepest part in the phase characteristics of the network 20. More precisely, the Q-value of the network 20 is related to the phase derivative, or group delay .delta..phi./.delta.f of the resonator according to: ##EQU1## where f.sub.o is the resonating frequency in Hz.
The output spectrum of the oscillator 5 can be determined by modeling the oscillator 5 as an extremely narrow band filter that filters the noise in the feedback loop. The noise power spectral density at the output S.sub.v, is thereby proportional to the whim noise density S.sub.n in the loop and inversely proportional to Q.sup.2 of the resonating structure according to: ##EQU2## where v=f/f.sub.o -f.sub.o /f.
When AM noise in the output spectrum is taken away in a known manner by applying an AGC or a limiter for example, only phase noise is left which can be modeled as FM modulation of a voltage controlled oscillator (VCO) 30 as illustrated in FIG. 2. As illustrated in FIG. 2, a non-ideal (low Q-value, noisy) oscillator can be represented by an ideal (infinite Q-value, noiseless) VCO 30 which has an equivalent noise source at its control input that produces the same noise spectrum at the output of the ideal VCO as the original, non-ideal oscillator would show. It is known to one skilled in the art of oscillator design to vary the frequency of an oscillator by controlling one of the components in the resonator. FIG. 3a illustrates an example of a parallel resonance circuit including a coil 40 and a capacitor 50. When the capacitance is made controllable by using a VARICAP or a varactor 55 as illustrated in FIG. 3b the resonance frequency at 1/(2.pi..sqroot.LC) is changed without affecting the Q-value of the resonator structure. The use of other resonator structures is possible as well such as structures with series resonance. By using narrowband FM analysis, the spectrum from equation 2 can be created from a whim noise source at the input of the VCO 30 having an equivalent noise voltage u.sub.n calculated by: ##EQU3## where K.sub.vco is the sensitivity of the VCO 30 in Hz/V.
When the oscillator is modeled as a VCO with a noise source at its input as illustrated in FIG. 2, the output phase noise can be reduced by applying negative feedback to the input of the VCO by measuring phase or frequency disturbances at the output of the VCO 30 and feeding these measured disturbances back to the input of the VCO 30. A known technique to compensate for phase disturbances is to lock the phase of a low Q-value VCO to a clean reference by the use of a phase locked loop (PLL) so that the low Q-value VCO will provide an output signal that is as pure as the reference within the PLL noise bandwidth. This can be considered as a negative PM feedback.
Another known technique for stabilizing the output of a direct digital frequency synthesizer is feeding back a demodulated output to the input of the direct digital frequency synthesizer as disclosed in U.S. Pat. No. 5,331,293 to Shepherd et al. This technique cancels out the jitter which corresponds to unwanted modulation caused by the operation of the digital synthesizer section. Also, a demodulator 118 develops a signal which is fed back to the clock for compensation as illustrated in FIG. 1. This compensation takes place on the clock 124 of the direct frequency synthesizer which is a VCO locked to a reference frequency by a PLL and this technique uses a complicated subtraction method for compensation.
Another known FM compensation technique is described in the text entitled "Communication Systems" by Carlson on page 270. In problem 7.3-2, Carlson discloses a direct FM generator including a VCO fed back with a signal derived from the FM detection of the VCO output. In this technique, the compensation removes DC offset and drift of the modulating signal by applying low pass filters in the loop.
Because it is desirable in many applications to increase the effective Q-value of an oscillator, careful design for the resonating structures of the oscillators is essential. Nevertheless, in chip integration, only low Q-value resonators can be processed. If there is a means to increase the effective Q-value of an oscillator, low Q-value resonators can be applied while producing high Q-value oscillators of low cost, high performance and high reliability.